An image forming apparatus such as copy machines or multi-function peripherals (MFP) may include a fixing device that fixes an image by heating a sheet to which a toner image is transferred.
Regarding the heating performed by the fixing device, a safety device is normally used in order to prevent an abnormal increase in a temperature. Such a safety device measures a temperature of an auxiliary heating member, and stops the heating when abnormal heating is detected. In the fixing device, a shielding member and the auxiliary heating member are physically separated from each other, so a thermostat used for measuring the temperature has access to the auxiliary heating member of which the temperature is to be measured.
In the aforementioned fixing device, because the shielding member and the auxiliary heating member are physically separated from each other, the thermal conductivity between the two members is low. When the shielding member and the auxiliary heating member are physically close to each other in order to improve the thermal conductivity, it is necessary to provide an opening in the shielding member for temperature measurement. The opening in the shielding member causes temperature variation in a fixing belt or in the auxiliary heating member.